If I were writing that sentence, I would use “in.”
That’s not the part of it that would raise a question with me, though. Instead I would be puzzled by this: “analyzed what were the most significant factors.”
To me this is ambiguous. It sounds like the significant factors were already identified before being analyzed. This is the same as just saying “analyzed the most significant factors,” so why not omit the unnecessary “what were”?
But in that case, how were they identified? The inclusion of “what were” makes it sound as if ascertaining the “what” were the purpose of the analysis. If identifying them is the result of the analysis, as seems more logical, then I think what you mean to say is “analyzed the factors in Lebron’s decision in order to see which were the most significant” or simply “identified the most significant factors in Lebron’s decision to join Miami.”
Either way, the wording lacks clarity and should be made more precise.